Machine Design

Thermoset Rubber, Structure of Rubbers

Originally, rubber meant the material obtained from the rubber tree Hevea Brasiliensis. Today, the term rubber means any materials capable of extreme deformability, with more or less complete recovery upon removal of the deforming force. Synthetic materials such as neoprene, nitrile, styrene butadiene (SBR), and butadiene rubber are now grouped with natural rubber. These materials serve engineering needs in fields dealing with shock absorption, noise and vibration control, sealing, corrosion protection, abrasion protection, friction production, electrical and thermal insulation, waterproofing, confining other materials, and load bearing.

Of all the available choices, SBR dominates the field, accounting for approximately one-half of all rubber -- natural and synthetic -- used in the U.S. The demand for SBR has been responsible for the building of a massive production capability for this material. More than half of SBR production goes into passenger-car tires in the U.S. Natural rubber is used almost exclusively in more demanding areas such as truck, bus, aircraft, and off-highway tires.

Structure of rubbers
In contrast with the ordered and rigid crystalline arrangement of atoms in metals, rubber atoms are arranged in long, chainlike configurations, which are in constant, thermally induced motion. The result is a tangled mass of kinked, twisted, and intertwined elements similar to a snarled fishing line. Along the chain, the atoms remain substantially the same distance apart, but the spatial distance from one point on a chain to another is always less than that measured along the chain's length.

Statistically, at a specific temperature, there is one most probable spatial distance between any two points on a given chain. When an applied force changes this distance, the thermal movement of the system sets up a force to restore the distance to what it was originally. This action accounts for the elasticity, or recovery, of a deformed rubber component. It also explains why the modulus of an elastomer, when heated, increases in the direction of strain.

Within the elastic solid, the tangled chain segments are relatively free to move with respect to one another, except to the extent that they encounter mechanical entanglement or, upon being vulcanized, are "hooked" together at chemically reactive sites on the chains and attain structural integrity.

TAGS: Materials
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